The invention relates to channel drop filters, and in particular to a highly efficient resonator-system channel drop filter. In exemplary embodiments, photonic crystal channel drop filters are provided.
The increasing interest in photonic integrated circuits (PICs) and the increasing use of all-optical fiber networks as backbones for global communication systems have been based in large part on the extremely wide optical transmission bandwidth provided by dielectric materials. This has accordingly led to an increase demand for the practical utilization of the full optical bandwidth available. In order to increase the aggregate transmission bandwidth, it is generally preferred that the spacing of simultaneously transmitted optical data streams, or optical data channels, be closely packed, to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized.
Channel dropping filters (CDFs) that access one channel of a wavelength division multiplexed (WDM) signal, and do not disturb the other channels, are essential components of PICs and optical communication systems. Among various devices introduced recently, resonant filters are attractive candidates for channel dropping because they can potentially be used to select a single channel with a very narrow linewidth. A schematic block diagram of a resonator-system CDF 10 is shown in FIG. 1, where two waveguides, the bus 12 and the drop 14, are coupled through a resonator-system 16 having one or more resonant cavities. The bus 12 includes an input port and output port, and the drop 14 includes a forward port and a backward port. While WDM signals (i.e. multi-frequency signals) propagate inside one waveguide (the bus), a single mode is transferred out of the bus and into the other waveguide (the drop), either in the forward or backward propagation direction, while completely prohibiting cross talk between the bus and the drop for all other frequencies. It will be appreciated by those of skill in the art that a channel drop filter is a filter which can transfer a channel from one waveguide to another, such as dropping a channel from one waveguide to another waveguide, or adding a channel from one waveguide to another waveguide.
The performance of a CDF is determined by the transfer efficiency between the two waveguides. Perfect efficiency corresponds to 100% transfer of the selected channel into either the forward or backward direction in the drop, with no transmission or back reflection into the bus. All other channels should remain unaffected by the presence of the optical resonator.
It is also desirable to obtain on/off switching functionality in the CDF. In other words, it is desirable to be able to switch on and off the transfer of a frequency channel. Ideally, in an "on" state, the frequency channel is completely transferred from the bus to the drop waveguide, while in an "of f" state, the frequency channel remains unperturbed in the bus waveguide. It is conceivable to achieve such on/off functionality using frequency tuning mechanisms. However, to turn off the device, the resonant frequencies of the coupling element must be shifted out of the entire frequency range of the signal. Such a frequency shift tends to require large switching power, and is not very practical.
Accordingly, it is an object of the invention to use an absorption-induced switching effect to achieve the desired on-off functionality.